This invention concerns self propelled vehicles of the type in which electrical drive motors are powered by an on-board batteries and control to drive vehicle wheels, and particularly where sophisticated controls are used as in automatically guided vehicles (AGV's).
A well known type of such vehicle is guided along a path to cause the vehicle to follow a buried guide wire. Steering is accomplished by control circuitry associated with each of a pair of DC drive motors drivingly connected to a respective driven wheel on either side of the vehicle to vary the drive of each motor to carry out the necessary steering of the vehicle so as to follow the buried guide wire. Many other automatic guidance schemes have been employed.
These vehicles also commonly employ fail safe braking, in which wheel brakes are spring applied, and electrically released when the vehicle is underway by application of a voltage to a suitable brake hold out actuator.
In the event of a shutdown of the vehicle for some reason, the usual result is immobilization of the vehicle since the brakes are set.
Thus, removal of the vehicle requires that it be hauled away as by a fork lift truck. This requirement sometimes creates difficulties in that getting a suitable recovery vehicle may be impossible in tight quarters.
The present inventors have discovered a further difficulty in manually rolling the disabled vehicle out of the system; that is, the DC drive motors act as electrical generators even at very low speeds, and the slight electrical currents so generated result in severe damage to the control circuitry associated with the motors.